1. Technical Field
The invention relates to pneumatic tires and in particular to pneumatic tires having sipes formed in certain of the tread blocks forming a pair of circumferentially extending ribs located on opposite sides of a mid-circumferential plane of the tire. More particularly, the invention relates to forming the sipes at angles in the tread blocks in a slanted forward direction in one of the ribs and in the opposite slanted reverse direction in the other of said ribs to make changes in the residual aligning torque (RAT) without changing the carcass or road contacting tread pattern of the tire.
2. Background Information
Pneumatic tires such as used for passenger and light trucks, have a tread pattern which extends circumferentially about the tire. The tread consists of a plurality of circumferentially and laterally extending grooves which divide the tread into generally circumferentially extending ribs formed by a plurality of either continuous or discontinuous tread blocks. Many of the tread blocks are formed with very narrow slots or slits commonly referred to as sipes. These sipes are formed by thin metal strips placed in the mold, referred to as sipe blades. These sipes provide various traction and stiffness characteristics to the tread blocks which provides certain ride and handling characteristics for the vehicle.
Even when the ribs of a tire are spaced symmetrical with respect to the mid-circumferential plane of the tire, they can produce a moment on the tread at the contact patch with the roadway, causing a twisting force on the tire. This moment provides a tendency for the vehicle to drift left or right when the steering wheel is released. This moment or force at the contact patch is referred to as residual aligning torque (RAT). The effect of this twisting moment and a detailed discussion relating to the various factors thereof and prior attempts to reduce or eliminate the same, is disclosed in detail in U.S. Pat. No. 5,944,082, the contents of which are incorporated herein by reference. The tire construction and method set forth in U.S. Pat. No. 5,944,082 provides draft angles in lateral grooves of certain ribs, which angles are sloped forward on one side of the mid-circumferential plane and sloped backward on the opposite side of the plane. These sloped grooves allegedly result in the tread blocks sloping forward and backward during forward motion of the vehicle which induces a torque to compensate for the inherent residual aligning torque due to the tread pattern.
U.S. Pat. No. 5,944,082 states that the siping in tread blocks is usually not wide enough to allow effective rolling radius changes before adjacent blocks come into contact and that the friction between sipes frequently locks the sipes together canceling the effect to be achieved. It concludes that inclined sipes on the rolling circumference of the rib are usually negligible and not predictable. However, the method of the present invention of forming the tire with oppositely sloped sipes has been found to achieve satisfactory results in affecting the residual aligning torque not believed possible in the tread pattern of U.S. Pat. No. 5,944,082.